1. Field of the Invention
The present invention relates to an electric power steering apparatus which uses an electric motor to provide power to assist the steering force for a vehicle.
2. Description of the Related Art
An electric power steering apparatus has been developed in which the speed of the vehicle and the steering torque applied to the steering wheel are detected and, when the detected torque exceeds a prescribed dead zone, a driving current, whose magnitude is determined according to the detected torque and vehicle speed, is supplied to drive a steering assisting electric motor whose rotating force is used to assist the force required for the vehicle steering, thereby providing a comfortable steering feel to the driver.
Such an electric power steering apparatus usually employs a direct drive motor as the steering assisting electric motor. The direct drive motor is suitable for use as a steering assisting electric motor since it does not require the use of a reduction gear and thus has the advantage of reducing the size of the motor-related hardware. In the electric power steering apparatus equipped with such a direct drive motor, a motor current controller sets a target current according to the detected torque and vehicle speed, and the direct drive motor is operated with three-phase alternating current supplied through an inverter according to the target current. However, the problem with this type of steering apparatus has been that the output torque of the electric motor is affected by a variation in the battery supply voltage as well as by the load change of the steering system, thereby causing an unnatural steering feel.
One previous approach to resolving this problem has been by current feedback control wherein the value of the three-phase alternating current for operating the motor is detected and the result of the detection is fed back to the motor current controller to control the three-phase alternating current in such a manner as to correct any deviation between the detected three-phase alternating current value and the target current value. More specifically, a current detecting resistor is inserted in an electric circuit path between the inverter and the electric motor; the three-phase alternating current value is detected using this current detecting resistor, and the thus detected three-phase alternating current value is dq converted by software (coordinate transformation of rotor windings to the static coordinate system), the dq current obtained as a result of the conversion being fed back to the motor current controller.
However, the above current feedback control of the prior art requires the use of a high-precision current sensor for detecting the three-phase alternating current value, but such a high-precision current sensor is large in size and not suitable for use in an electric power steering apparatus where miniaturization of hardware is a desired condition. Another problem with the prior art is that the complicated feedback control involving dq-converting the detected three-phase alternating current value increases the complexity of the software. This has created the further problem that the increased complexity of the software decreases the control response.